The dark days of the financial crisis cast a long shadow over a key stock-market measure

In math, how you get the answer to a problem can be as interesting as the answer itself. For financial-market ratios that grab investors’ attention, it may be even more important.

Take the oft-cited cyclically adjusted price-earnings ratio for U.S. stocks calculated by economist Robert Shiller. The ratio stands at 31.3, a level only seen before in 1929 and around the dot-com bubble—a potential source of concern for investors worried about a rerun of history.